Update mac os targets from 10.14.3 to 10.14.4, .5 and .6

in order of how many of each we have in the pool.

Bug: 947561
Change-Id: Ia8d94e0c009d1bcb3b9142ae172e661f9f764c93
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1796523Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Lindsay Pasricha <lindsayw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695691}

testing/buildbot/chrome.json

@@ -2364,14 +2364,14 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Mac-10.14.4",
+              "os": "Mac-10.14.6",
               "pool": "chrome.tests"
             }
           ],
           "optional_dimensions": {
             "300": [
               {
-                "os": "Mac-10.14.3"
+                "os": "Mac-10.14.4"
               }
             ],
             "600": [

testing/buildbot/mixins.pyl

@@ -364,14 +364,14 @@
   'mac_10.14': {
     'swarming': {
       'dimensions': {
-        'os': 'Mac-10.14.4',
+        'os': 'Mac-10.14.6',
       },
       'optional_dimensions': {
-        # Wait 5 minutes for 10.14.3, then 5 for .5, then fall back to 10.14.4
+        # Wait 5 minutes for 10.14.4, then 5 for .5, then fall back to 10.14.6
         # The format for optional dimensions is: expiration: [{key, value}, ..].
         300: [
           {
-            'os': 'Mac-10.14.3',
+            'os': 'Mac-10.14.4',
           },
         ],
         600: [